1. Field of the Invention
This invention relates to the field of prostheses for use in the replacement of at least a portion of a defective hip joint.
2. Description of the Prior Art
In the use of prostheses for the replacement of hip joints there have been problems of failures in these hip systems. There are tremendous forces exerted on the prostheses and the related bone structures during activity of the patient. Thus, in jumping from a 2-3 foot elevated surface to a lower surface the loads placed on the hip joint can, for example, be many times the body weight, and for higher elevations the load factors will be greatly multiplied. The abovementioned failures can occur in the breakdown of the bone, in the cement, and in the femoral prosthesis itself.
Before the introduction of the use of cement in the hip systems many different femoral component designs were utilized, and with the advent of the use of cement in such systems the femoral component, having sharp edges, would cut into the cement, causing cracking and other problems. Also, such prior devices had depressions or undercut portions, which when the cement hardened would lock the femoral prosthesis in place against removal so that it for some reason another operation were necessary to remove the prosthesis and replace it, then it would be difficult for the prosthesis to be removed.
A major disadvantage of the femoral components of prior hip joint prostheses occurs when, after the surgeon has prepared the femur to receive a certain normal-size femoral component (i.e., normal for the particular size and bone structure of the patient) by reaming and broaching the shaft of the femur, he discovers by trial and error or the like that the use of that specific femoral component will result in a hip joint that is too tight or too loose. This may be due to the abnormal structure of the patient's hip joint and surrounding bone structure, the deterioration that has occured to the patient's hip joint, or the like. The prior femoral components were adapted in two known ways to adjust the tightness of the hip joint. First, some prior femoral components are made with neck portions of different lengths while maintaining the same shape and size of stem and head and the same angle between the neck portion and the stem. While the use of such components does result in varying the tightness of the hip joint, it also causes the shaft of the femur to be displaced from the normal position as provided by a normal healthy hip joint which results in more stress on the hip joint and surrounding areas, and the like. Second, some prior femoral components are made with stems of different curvatures to thereby vary the angle between the shaft and the neck portion while maintaining the same length neck portion and same size head. While the use of such components does result in varying the tightness of the hip joint, it also requires the surgeon to reprepare the femur to receive the specific curvature of the stem of the proper component which results in lengthening the time required to complete the operation thereby causing the patient more trauma and the like, often results in weakening the shaft of the femur since more bone is removed than would have been absolutely necessary if the curvature of the stem had not changed, and also results in displacing the shaft of the femur from the normal position as provided by a normal healthy hip joint.
Some of the prior art devices are shown in the following U.S. Pat. Nos.: 3,918,102; 3,965,490; 3,922,726; 3,863,273, and D-235,485. Also, there is a prior device known as the "Charnley Total Hip" which has a stem portion that is flat on the posterior and anterior surfaces, is substantially uniform in thickness as measured in the A-P (anterior-posterior) direction, and has a neck portion circular in cross-section. Also, among the prior art devices is total hip system known by the trademark "Protasul". None of the above prosthetic devices disclose, teach or suggest the present invention.